The smartass reply is it'll work until it doesn't. When you are running on ice or sufficient snow, you can get by, but when the mis-matched tires are on good traction, you'll start stressing the differentials when in 4wd.
The sincere reply is buy two additional Cooper's (or Cooper sized). If you buy rims, it is easy-peasy to switch them out.

 

First, I'm assuming a full size truck. You can get away with quite a bit at lower speeds in 4x4. I'm calling lower speeds 50 or so.

You'll notice that most diesel trucks bulge the front tire more than the rear due to the extra weight up front. Similarly, you can see trucks with the rear bulging more than the front. If you measured from the center of the hub, I think both cases the bulging tire is significantly shorter. I haven't measured, but I wouldn't be surprised if there isn't a 1/2 inch difference to center of hub. That translates to about an inch overall diameter difference. YMMV. I would measure loaded to see how much difference you actually have. The other potential issue is handling difference due to tire construction/tire pressure although most trucks don't handle that well any way.

In any case, some slipping will be occurring when you actually need 4x4 which will make up some difference as long as you don't have lockers front and rear engaged.

Different rules apply with small diameter tires with full time all wheel drive. Some vehicles have significant issues like the Audis from what I've heard.

 

I'm getting away from the Coopers, never thought the tread was aggressive enough. These two are the leftovers from getting the AT's, mounted on wheels. Never ran them all the time, usually on return trips from MT and ND, all highway. They're good tires. Just wondering how much wiggle room (if any) I have. Will sell them if needed. I would think a two inch difference would be excessive.

 

They would sell fast I bet .How your spare tire , some rotate the spare into the set .If spare is used up , use one of the new coopers.We all know running on ice we don’t want any surprises .

 

Jim: Full size 3/4 ton Duramax. Wondered about the front tire bulge. Heard years ago the front was geared different than rear so it is always pulling the vehicle ahead and the rear doesn't try to pass. A vehicle would be undriveable on the slick stuff, no control what so ever. Dunno how true that is. It does have locking differentials which is why these trucks eat tires.

If the Coopers are larger than the AT's by any amount, I won't run them at all.

 

Surprises... That's the idea.

I gotta drop the spare, thanks for the reminder. Can't remember how long its been under there. Too long I'm sure. Truck is going on 19 years old, 2005 purchased in 2004. 130,000 miles, still new.

 

Could have the G80 Rear Diff .Nice option 😳

 

Very few vehicles have Factory full lockers ,some have Rear G80 , E locker , Air Locker .Usually factory setup for low range (low speed).Eaton G80 works when one wheel RPM is a little higher .
A few vehicles have the Front Locker like the ZR2 ,they seem to work fine ,but low Range only ,and steering becomes a little stiff .
Detroit lockers and spools have no give /slip ,so you go chirping in the turns .

Subis have an interesting drivetrain , but not locked up or full QuadTrack.

 

If you are in a situation that you need 4wd, it likely won't matter. They can slip on snow/ice/mud, binding of the drivetrain won't be an issue. What we don't know is the difference in tire size. You mention two inches, is that in relation to your tires? Two inches in height, two inches in circumference? Both would be significant.

What I would be FAR more concerned with in your 4wd system is whether or not pump rub has been addressed in the transfer case. A pinhole can spew all of the fluid out of the case in 2000 miles, and it won't leave a drop on the ground where you park it.

 

Two inches in circumference. To me, that's a lot.

"Pump rub." Completely new term to me. My concerns mostly had to do with the transfer case, not the differentials.

 

What ever you do put the larger ones on the front. I know several off road guys that do this so the front will pull more and they will turn. Put the larger ones on the rear and the front will push and you will loose steering in the loose stuff.

 

Two inches really isn't that much, certainly nothing I'd worry about on ice or snow. That's a minimal difference in tire size, about like going 275/75/16 to a 285/75/16, a small difference (32.2 vs 32.8" tall). With BGF ATs, I would just sell the Coopers anyway, unless you drive on a lot of ice. KO2s are great in the snow.

Pump rub is common, and it will destroy your transfer case if not detected and repaired. It affects 98-07 GM half tons and 01-07 HD trucks. The tabs that hold the pump inside the transfer case wear, which allows the pump itself to rotate and come in contact with the housing. This eventually wears a hole in the top of your transfer case, and fluid is pushed out that hole, but only while driving. They rarely leak enough to drip down the case to leave a spot on the ground. Sometimes there's no indication anything is wrong, but often you will see small oil spots on your tailgate or trailer. I have seen many fail between 60K and 100K miles, although some (like my 01) didn't wear a hole until 300k. While I was being lazy and waiting to repair mine, I would have to add over a quart of fluid every 1000 miles to top it off, and that was a small pinhole. They only hold 2 quarts. The repair is not something I would put off, regardless of miles, the kit is about $75 and only takes a couple hours to install, depending on how thirsty you get during the process.

The front differential and rear axle in those trucks are just this side of indestructible.

 

Tire bulge doesn't change the circumference of the tire, it's the same distance every rotation, bulge or no bulge right? I wouldn't run different size tires with full time selectable 4x4 transfer case, something may break, when you shift into 4wh drive. . I used to run Coppers on my dually but could on get about 28k wear, change out after 3 sets. 6 tires mounted, balanced, taxes, 2400.00 out the door was getting to be too much for 28k miles. They were the Cooper ST Maxx, loved the look though,,,gregg

 

Tire bulge doesn't change the circumference of the tire, it's the same distance every rotation, bulge or no bulge right?

Incorrect. Revs per mile actually changes loaded vs unloaded.

 

Dog, I say you’re ok if using this for low-traction application only. 2” circumference is what, 5/8” diameter difference? Not perfect, but should be fine in low grip situations like snow and ice. Ideal? No.

 

I’m with boater and Willie… I don’t see how the circumference would change much when loaded heavier. If you take a string of fixed length it shouldn’t matter if it’s shaped like a circle or like a caterpillar track, rolling it along the ground it should take the same number of revolutions per unit distance. I can imagine maybe a little change from changing the amount of swelling due to inflation, not sure how much the circumference changes with inflation.
But… I could be wrong!

 

I’m with boater and Willie… I don’t see how the circumference would change much when loaded heavier. If you take a string of fixed length it shouldn’t matter if it’s shaped like a circle or like a caterpillar track, rolling it along the ground it should take the same number of revolutions per unit distance. I can imagine maybe a little change from changing the amount of swelling due to inflation, not sure how much the circumference changes with inflation.
But… I could be wrong!

You are correct. The real-world term is "rolling radius". It may change a little bit for under inflated tires, but not much. Back when I was calculating truck performance in engineering classes we would take the tire's listed spec and use it for all calcs. If you do have a big bulge in a loaded tire, it's underinflated or overloaded. That bulge means that the tire is flexing that much on every revolution, causing it to heat up and potentially fail. A slight bulge is expected, but not very much.

As to the original question, sell the Coopers. Differing tread wear is enough to cause binding in the 4x4 system. Even 1/8" difference in tread depth between front and rear makes my trucks very difficult to shift out of 4x4.

 

Circumference won't change, but that's not important.

My explanation which could be wrong. The question is will the gears turn faster or not. That is more impacted by diameter if you consider there is a center point of the contact patch. In other words, all of the tire isn't making contact at one time, only "one" point is.

 

Circumference doesn't change but diameter does. Measure from center of hub to surface and again from center to top of tire.

 

It really should be noted that all wheel drive and good old 4 wheel drive work a little different, but my guess is all wheel drive 4x4s would most likely be damaged much easier then the old style 4 wheel drive. In either case driving with miss matched tires is a crap shoot because of the way 4 wheel drives work. The tires with the least traction always get the most power and a smaller tire is going to move faster then a bigger tire and appear like it is losing traction at least that is my understanding of the issue and why 4 wheel drives always have a full size spare and passenger vehicles have space savers. Of course these days they might not even supply a spare with new cars for all I know I drive old clunkers lol

 

When trying to understand these 'engineering' principles (what does underinflation due to the distance travelled per revolution), I find it makes it easy to conduct a thought experiment at the limits of the equation. We do this all the time in engineering: in fact, most calculus is defined by the limits of the equations. So anyhow, all it needs is a simple thought experiment at some of the far limits to understand:

Pretend you have a wheel of diameter 2". And you've attached a super-underinflated, super-thin tire that would normally be 10 foot diameter when inflated. But because it is super-underinflated, the tire is mashed against the 2" diameter wheel at the road surface. As we slowly rotate the wheel/tire, we see it has an effective diameter of 2" - not the 10 feet diameter that the tire is capable of with full inflation. So for every revolution, the effective circumference is 2*pi*r = 2*pi*1" = 6.3" distance travelled. If we then fully inflated our 10' tire so it was indeed round (instead of oblong), we would get the effective circumference to be 2*pi*60" = 378" distance travelled per revolution. So what was getting people confused here is comparing the circumference of a circle to the circumference of an oblong form and thinking that the shape had no effect: it has huge effect, potentially, in the distance travelled per revolution.

This would be a great interview question. Always interesting to see how people attack solving problems like this.

 

It’s easy to prove, take a 3 or 4wheeler with a solid rear axle. Set one rear tire to 3 psi, the other to 15. Go for a drive, it’ll pull hard to the low side. The rev/mile of that soft tire is less than that of the overinflated one.

To get back to the thread topic, I’d say put the better tires on the front, use 4wd only when it’s slick and you need it, enjoy life. This applies to 4wd rigs that normally run in 2wd. Not awd or full time 4wd rigs.

 

Or is it the resistance of the underinflated tire of 5:1 psi to the opposite tire? We had Cat 988 loaders in the quarry's when I was still working. Per Caterpillars maintenance schedule, the shop mechanics would periodically measure tires circumference's, they would change them if there was too much difference, even if there was a lot of good tread left on a tire. I didn't get into to many details with them about it, but it had to do with front turning at a different rate than the rear. I would not run tires of much difference with a transfer case that was chain driven, or gear, there's no slip accounted for in them,,,gregg

 

I only use 4 when I need it and ty not to stay in for very long. Like Mark said in another thread a while back, don't travel in the Dakotas or MT without studs in the winter. So far, I haven't. I'm so disappointed the AT's can't be studded. Maybe I need to pick up a couple more wheels and buy a complete set of something else that can. Any suggestions as to what? The problem with buying tires here is they might not be suitable where I go. Might be something better available in MT?

 

Might be something better available in MT?

I live in MT, and we've had very good luck with siped 3 Peak tires in Winter on our truck and SUVs. Obviously, real Winter tires with studs could be better, but we keep the same tires on year round.

My MIL lives down in the valley and doesn't have to go up any hills. She drives a Camry with my recommended all 4 studded Winter tires in Winter which does fine. Sure, she has to wait until the roads are plowed if there is very much snow due to her car level ground clearance.

Studded true Winter tires are pretty amazing.